There is the (relatively) difficult way to tweak the settings, which involves pushing some commands over to your device over ADB. Many Nexus 5 users likely wouldn't have an issue doing that, but most casual consumers might be put off by having to install the Android developer tools and using the command line to tweak settings. Luckily, there is also a much easier way to tweak the setting: with an app.
All you'd have to do is install the app, run it, reboot your device, and you're done. You can even uninstall the app after that because it is tweaking a setting within the system and the the app is unnecessary after that. The only downside to the tweak is that the battery percentage is hard-coded to be white... on a white battery meter. So, it can be kind of useless until you get past 50%. The color could theoretically be changed, but a tweak like that would cause future OTA updates to fail, so it's not worth the effort for now.
If you want to give it a try, the APK links are at the source.